We have reason to believe the two latest Netsky variants are not written by the original viruswriter behind this family. As he earlier claimed to stop distributing new variants and instead release the source code of the worm, this might be exactly what happened. After that, third parties have modified the source code and released new variants based on it.

This is just speculation, and we're not sure if the source code of Netsky has been posted publicly. At least we haven't seen it.